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(1) Field of the Invention
This invention relates to a process for inhibiting scale and fouling on metal surfaces exposed to an aqueous system, particularly a circulating aqueous system. The process comprises adding glutamic acid N,N diacetic acid, or salts thereof, to the aqueous system. Glutamic acid N,N diacetic acid, or salts thereof, act as a chelating agent and is biodegradable. The process is particularly useful for inhibiting the formation of scale and fouling on metal surfaces of steam generating and cooling systems.
(2) Description of the Related Art
Divalent and trivalent cations, e.g. calcium, magnesium, iron and copper, are often found in the water, which circulates in various heating and cooling systems, for example chillers, boilers, and process heat exchangers. Typically, the heating and cooling system is comprised of components or equipment made of metal such as iron, steel, aluminum, etc. The divalent and trivalent metal cations often form a precipitate in the presence of anions, e.g. sulfate, carbonate, silicate, and hydroxide found in the water and form scale on the metal surfaces of the parts of the heating and cooling system.
It is known to add salts of ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) to the water circulating through heating and cooling systems. These salts react with the divalent and trivalent cations to form soluble, thermally stable complexes, which reduce or eliminate the formation of scale and fouling on the metal surface of the metal components and equipment used in the heating and cooling systems. In steam generating systems, the complexed cations include calcium, magnesium, iron and copper.
There are problems with using the salts of ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) in aqueous systems to chelate metal cations. Regulations in Europe require that the blowdown of boiler water must contain materials that are readily biodegradable. Thus, EDTA is not acceptable since it is not biodegradable. Although NTA is biodegradable, it is classified as a carcinogen in the United States.
There is an interest in discovering biodegradable materials that are thermally stable, yet have anti-precipitation capabilities comparable to EDTA, when used in a heating or cooling system where the circulating water contains calcium, magnesium and/or iron cations.
It is known that from the product literature of Contract Chemicals that NERVANAID GBS5, which contains glutamic acid N,N diacetic acid tetrasodium salt as an active ingredient, is a biodegradeable sequestering agent for household and industrial and institutional detergents.
All citations referred to under this description of the xe2x80x9cRelated Artxe2x80x9d and in the xe2x80x9cDetailed Description of the Inventionxe2x80x9d are expressly incorporated by reference.
This invention relates to a process for inhibiting scale and fouling on metal surfaces exposed to an aqueous system, particularly a circulating aqueous system. The process comprises adding glutamic acid N,N diacetic acid, and salts thereof (e.g. the tetrasodium salt), to the aqueous system. Preferably used is glutamic acid N,N diacetic acid tetrasodium salt, which acts as a chelating agent. It chelates metal cations, particularly calcium, magnesium, and, to a lesser extent, iron. The process is particularly useful for inhibiting the formation of scale on metal surfaces of industrial, commercial and institutional water systems, particularly boilers operating at temperatures of 120xc2x0 C. to 270xc2x0 C., and moderate pressures, up to 750 psig (50 bar).
The glutamic acid N,N diacetic acid tetrasodium salt is biodegradable and exhibits thermal stability when present with cations. The anti-precipitation capabilities are similar to EDTA with respect to calcium and/or magnesium cations. Thus, glutamic acid N,N diacetic acid tetrasodium salt could replace EDTA.Na4 as a chelant in aqueous systems. Experiments suggest that glutamic acid N,N diacetic acid tetrasodium salt is more effective in chelating calcium cations (whether the calcium cations are present alone or in combination with magnesium and/or iron cations) and in inhibiting calcium scale in aqueous systems circulating through boilers made of steel than EDTA.
Not Applicable.
The detailed description and examples will illustrate specific embodiments of the invention will enable one skilled in the art to practice the invention, including the best mode. It is contemplated that many equivalent embodiments of the invention will be operable besides these specifically disclosed.
Preferably used is glutamic acid N,N-diacetic acid tetrasodium salt is represented by the following structural formula: 
The glutamic acid N,N-diacetic acid tetrasodium salt is added to an aqueous system such as cooling water, boiler water, reverse osmosis and geothermal/mining water in amounts from 1 to 500 ppm, but preferably from 10 to 50 ppm.
The process is particularly useful for aqueous systems circulating through boilers made of steel, although the process is useful for aqueous systems circulating through equipment made of other metals, e.g. iron, aluminum, brass, copper, and alloys thereof.
The glutamic acid N,N-diacetic acid, or salt thereof, may be combined with other components used in scale inhibitor compositions, e.g. corrosion inhibitors, surfactants, dispersants, precipitants, antifoams or agents that inhibit microbiological growth.
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